“Double block and bleed” is a well-known term of art that refers to setting two seals in a pipe and opening a bleed port between the seals to ensure that the first seal is holding. Any leakage past the first seal is contained by the second seal and forced to exit through the bleed port. This arrangement ensures that the pipe is completely sealed, making it safe to work on downstream of the two seals.
Achieving double-block and bleed typically requires the use of a single, metal-to-metal plugging seal or the use of two independent pluggers and fittings. Both approaches are expensive; use relatively large, heavy components; and require a lot of equipment to properly install. Additionally, the metal-to-metal plugging seal approach does not allow for bypassing pipeline product through a fitting. If a bypass is required, a second fitting must be installed upstream of the plugging seal.
U.S. Pat. No. 7,270,139, issued to Calkins et al. on Sep. 18, 2007, discloses a cam-assisted, wedge actuated, double block and bleed, metal-to-metal plugger. A pair of wedge members is forced against a rearward surface of the metal sealing cups to simultaneously force the cups into metal-to-metal sealing contact with the sealing surfaces formed by opposing toroidal collars welded to the exterior of the pipe. Because of the metal-to-metal seal, requires that each collar have precise face surfaces and that each collar be precisely welded onto the pipe and accurately spaced in relation to one another. The plugger cannot seal against the cut and relatively rough ends of a pipe.
Another style of double block and bleed plug that eliminates many of the disadvantages of typical double block and bleed pluggers is disclosed in U.S. patent applications Nos. 11/936,182 and 12/256,150, filed by Yeazel et al. on Nov. 7, 2007 and Oct. 22, 2008, respectively. The plug includes pivotally connected plugging heads which work in combination to seal against an interior wall surface of the pipe downstream of the access opening to the pipe. The first plugging head prevents flow of product in the pipe and the second plugging head captures any leakage past the first head and forces that leakage out through a fitting. The plug may include standard sealing elements or an inflation system used in combination with inflatable sealing elements or compression packer sealing elements. Although the plug does not require a precise sealing surface, the plug is not configured to seal against the cut ends of the access opening to the pipe.